Acids, Bases and Salts: Class 10 short notes
This chapter is a favourite for one-mark and assertion-reason questions. Most marks come from three things: what each indicator does, the pH scale, and the five salts with their formulas and uses. All three are below, in short.
Short notes
- Acids taste sour and turn blue litmus red. Bases taste bitter, feel soapy and turn red litmus blue.
- Phenolphthalein is colourless in acid and pink in base. Methyl orange is red in acid and yellow in base.
- Olfactory indicators change smell: onion, vanilla essence and clove oil lose their smell in a base.
- Acid + metal → salt + hydrogen gas. Hydrogen burns with a pop sound.
- Acid + metal carbonate or hydrogencarbonate → salt + carbon dioxide + water. CO₂ turns lime water milky.
- Acid + base → salt + water: this is neutralisation.
- Acids give H⁺ (as H₃O⁺) ions in water; bases give OH⁻ ions. Bases that dissolve in water are called alkalis.
- Always add acid to water slowly, never water to acid: the mixing gives out a lot of heat and can splash.
- pH runs from 0 to 14: below 7 is acidic, 7 is neutral, above 7 is basic. The lower the pH, the stronger the acid.
- pH in daily life: the stomach makes HCl, and antacids such as milk of magnesia neutralise extra acid. Tooth enamel starts to decay when the mouth's pH falls below 5.5. A bee sting or nettle sting injects methanoic acid; a mild base like baking soda helps.
- Chlor-alkali process: electrolysis of brine (NaCl solution) gives NaOH, chlorine at the anode and hydrogen at the cathode.
- Water of crystallisation: blue copper sulphate crystals (CuSO₄·5H₂O) turn white when heated, and blue again when water is added.
At a glance
| Salt | Formula | Made from | Used for |
|---|---|---|---|
| Bleaching powder | CaOCl₂ | Chlorine + dry slaked lime | Bleaching, disinfecting water |
| Baking soda | NaHCO₃ | NaCl + H₂O + CO₂ + NH₃ | Baking powder, antacid, fire extinguishers |
| Washing soda | Na₂CO₃·10H₂O | Recrystallising sodium carbonate | Cleaning, removing hardness of water |
| Plaster of Paris | CaSO₄·½H₂O | Heating gypsum at 373 K | Plastering fractures, toys, decoration |
| Common salt | NaCl | Sea water, rock salt | Raw material for the salts above |
Memory tricks
Litmus: BRA
Blue turns Red in Acid. The opposite (red to blue) is a base.
Phenolphthalein goes pink for a base
Think "Pink Bubble-gum Base": pink only in a base, colourless in an acid.
Plaster of Paris is half
Gypsum has 2 water molecules; heat it and Plaster of Paris keeps only ½. Add water and it sets back into gypsum.
Practice questions
A solution turns red litmus blue. Its pH is likely to be:
Show answer
10. Red litmus turns blue in a base, and bases have a pH above 7.
The chemical formula of Plaster of Paris is:
Show answer
CaSO₄·½H₂O. It has half a molecule of water per CaSO₄.
Which gas is produced when dilute HCl reacts with zinc granules?
Show answer
Hydrogen. Acid + metal gives a salt and hydrogen gas.
Tooth decay starts when the pH in the mouth is lower than:
Show answer
5.5. Below 5.5, tooth enamel (calcium hydroxyapatite) begins to corrode.
Questions
What is the difference between a base and an alkali?
All alkalis are bases, but only bases that dissolve in water are alkalis, such as NaOH and KOH.
Why does dry HCl gas not change the colour of dry litmus paper?
Without water, HCl can't release H⁺ ions, and it's the H⁺ ions that make it act as an acid.
Can TellBrief make notes from my own chapter PDF?
Yes. Add the PDF or photos of your book's pages and choose CBSE once: you get short notes, memory tricks and board-style questions, each linked to its page.